1. Field of the Invention
The present invention relates to an image pickup apparatus that incorporates a solid-state image pickup element that can be driven by splitting a charge transfer gate, which controls the transfer of a charge from an image part that makes up a pixel array to a vertical transmission path, into a plurality of phases.
2. Description of the Related Art
In recent years, there has been much development of digital still cameras (hereinafter referred to as electronic cameras) that obtain video signals by picking up the object image using a solid-state image pickup element such as a CCD. Various designs have been conceived for driving this image pickup element. In one of these, a charge is transferred by staggering TG pulse timing.
More specifically, when a mechanical shutter is not used, the charge storing in the image pickup element ends with the output of the aforementioned TG pulse, which transfers a charge from the photoelectric conversion storage part (pixel part) to a vertical transmission path. Accordingly, it is generally preferable that TG pulses be output simultaneously for all pixels and in most cases control is implemented to enable this.
However, depending on the capabilities of the image pickup element, when a TG pulse is applied to all pixels simultaneously, a reverse charge injection phenomenon, caused by major fluctuations in substrate potential in areas around the pixel part, arises from the semiconductor substrate. This in turn causes false signals. One means of countering this anomaly is to split the TG drive line into a plurality of phases and stagger the timing of TG pulses for each phase by the minimum time (by only a few 10 μs).
In addition to this, another known method that is implemented in order to read neighboring pixels of the same color after adding them together without mixture with other colors using a color image pickup element that is color-coded in a Bayer array, for example, is a so-called “pixel realignment addition and reading”, in which the timing of TG drive lines that have been split into a plurality of phases is staggered and the lines then sequentially driven, and a number of pixels (number of lines) are vertically transmitted.
The issue raised when the timing of TG pulses is staggered using phases for various purposes, is that differences in the exposure times in each phase occur equating to those time differences. The differences in exposure appear as anomalies such as lateral stripes (intensity difference among horizontal lines) in the image. A known method of countering this anomaly involves compensation that varies the signal gain to suit the amount of exposure (exposure time). In concept, this is a compensation involving multiplication.
The implementation of analogue gain compensation (multiplication) for each line corresponding to each phase as above is frequently accompanied by an increase in the scale of the hardware and anomalies such as faults arising from line instability. For this reason, multiplication is used for digital signals after the signal output from the image pickup element has been converted into digital format. However, various new problems arise. These are: (1) sometimes calculation processing time increases and processing cannot be completed within a prescribed time; and (2) sometimes, depending on the calculations involved, the effect of bit errors (quantization errors) increases and adequate compensation is not performed.